1. Field of the Invention
The present invention relates to a filter circuit used as a basic circuit when processing analog signals in a MOS integrated circuit (IC).
2. Description of the Related Arts
In recent years, due to the increase in digital devices and advances in digital signal processing technology, CMOS integrators applied for digital signal processing account for a large portion of the semiconductor market.
However, since video and audio signals have analog input/output, they can be more easily processed by analog processing. Even when video and audio signals are digitally processed, analog circuits are required for A/D and D/A conversion and filtering carried out before or after conversion and in a clock-generating oscillator and such like. Conventionally, bipolar technology has been regarded as suitable for analog circuits, whereas CMOS technology is regarded as unsuitable except for some circuits such as analog switches, sample holders or such like.
However, bipolar and BiCMOS processing are rather expensive, and in view of the strong demand for a 1-chip CMOS structure achieved by digital/analog consolidation, there has been an increase in development of circuits for processing analog signals with CMOS circuitry.
Analog signal processing features an important function known as an xe2x80x9cactive filterxe2x80x9d which has a high frequency of use and exerts considerable influence on total performance. Conventionally, discrete-time processing filters such as switched capacity filters (SCF) or sample data filters have been the main active filters used in CMOS analog technology. While these filters have the advantage of high precision, since frequencies are fixed precisely according to a clock, and have low sensitivity to capacitor variations originating during manufacturing, these filters have the following disadvantages:
1. continuous-time filters are required before and after due to the existence of xe2x80x9caliasingxe2x80x9d;
2. since a frequency band several times greater than frequency used for operational amplifier and sample-and-hold (S/H) circuit is required, the filter cannot be used for frequencies higher than the video band.
3. circuit scale is large and therefore not economical. Therefore, it is not possible to produce a simple and inexpensive filter which can be used at high frequencies. This problem cannot easily be resolved since it derives directly from the fact that the filter is a discrete-time filter. Recently, attempts are under way to develop a high-performance continuous-time CMOS filter. The most popular continuous-time filter is a xe2x80x9cbiquad circuitxe2x80x9d, consisting of two integrators each comprising a transconductor (Gm) circuit and a capacitor, in which a second-order filter is multistage-connected in order to obtain desired filtering properties. In a bipolar technology, transconductor (voltage-current conversion characteristic) is linearized its characteristic using a resistor and a xe2x80x9cgain cell structure of transistorsxe2x80x9d. However, when the above method is used in CMOS, since element Gm (transconductance) is small, many elements of enormous size are required, which is extremely uneconomical. Transconductance properties are therefore obtained by performing voltage-to-current conversion using a differential pair-transistor direct-coupled to the source.
However, in a MOS transistor, gate-source voltage versus drain current is a square relationship and the integration property of capacitor current versus voltage is linear, a single output of the integrator has second-order distortion. It is therefore necessary to cancel second-order distortion using a fully differential input and output signals. In such fully differential processing, since the output DC voltage cannot usually be determined, a bias circuit must be added to determine the DC operating point. Generally, therefore, output DC voltage is detected and DC feedback technique is applied to the bias of the transconductor input. This method is called DC feedback (or common-mode feedback or such like).
FIG. 6 shows a conventional example of this type of fully differential biquadratic circuit provided with DC feedback. This circuit is made up of integrators in two stages, each stage consisting of two transconductors sharing a common output terminal and a capacitor connected to said output terminal. Lower transconductors Gm1+ and Gm2+ correspond to + input in a single configuration; upper transconductors Gm1xe2x88x92 and Gm2xe2x88x92 correspond to xe2x88x92 input (feedback input) in a single configuration.
Two-stage integrators are cascade-connected as above to form a low-pass filter or such like. The output DC voltage is controlled to a predetermined voltage by monitoring the output of DC feedbacks 1 and 2 at each integrator and controlling the bias current of the output terminal in each stage. In addition to an LPF, other filter types such as BPF and HPF can be created by modifying the signal input position and output signal extraction position.
FIG. 7 shows a concrete example of integrators in the stages forming the filter in FIG. 6 realized using CMOS circuitry. A pair of source-coupled differential transconductors are used to produce transconductance. The differential circuit comprising M1, M2 and I1 corresponds to transconductor Gm1+ or Gm2+; the differential circuit comprising M3, M4 and I2 corresponds to transconductor Gm1xe2x88x92 or Gm2xe2x88x92. The current outputs are summed at the output point. Output is differentially extracted by biasing with two current supplies I3, I4 which join at GND. With respect to differential input bias currents I1 and I2, these current supplies I3, I4 must be precisely (I1+I2)/2 respectively. If this relationship is evenly slightly disrupted, since the DC impedance of the output terminal of each stage is extremely high, the unbalance between the upper and lower current supplies causes the output DC voltage to be greatly disrupted, leading to instability.
xe2x80x9cDC feedbackxe2x80x9d is a circuit designed to counter this problem, ensuring stability by fixing output DC potential at a certain voltage. One of the resistor terminals is connected to the output terminal in FIG. 7. The other resistor terminal is now connected to each other, and is compared by the operational amplifier with the intended voltage Vref. When all output signals are fully differential and the two resistors are equal values, the DC potential of the output signal can be extracted from the center point and compared with Vref. When the DC potential is higher than Vref, the current from the current supply is increased and the common-mode voltage of the output is lowered. Conversely, when the DC potential is lower than Vref, the current from the current supply is decreased, raising the common-mode voltage of the output.
Thus current is controlled so that the common-mode voltage of the output signal equals Vref. The circuit is biased so that the DC voltages of differential outputs in each stage of the filter equal Vref, but the number of circuit elements is likely to be increased. Not only does the operational amplifier itself require a considerable number of elements, but a buffer circuit and such like must be provided as shown in the diagram in order to prevent the resistors which detect center point voltage from influencing the high-impedance integrator output terminals. Moreover, a DC feedback circuit is required for each integrator, thereby taking up a large portion of the overall area of the filter.
In the circuit shown in FIG. 7, for instance, the essential portion of the integrator within the dotted line on the left side comprises 4 MOS transistors, 4 current supplies and 1 capacitor, thus requiring approximately 10xcx9c15 elements. By contrast, a DC feedback circuit for setting bias has as many as 20xcx9c30 elements, thereby taking up two thirds of the area. Since the filter is formed simply by assembling these elements, it follows that the DC feedback circuit takes up roughly two thirds of the total filter area. This need for a DC feedback has resulted in increased costs for fully differential filters and has been an obstacle for producing an inexpensive filter.
An object of the present invention is to provide a fully differential filter circuit which does not require DC feedback circuit.
One aspect of the present invention is to provide a filter circuit comprising a first circuit block, having a first differential-mode voltage input and a differential-mode current output, and a common-mode voltage of the first differential-mode voltage input being inverted at the output, and a second circuit block, having a second differential-mode voltage input and a differential-mode voltage output, and a common voltage of the second differential voltage input being inverted at the output, wherein the first and second circuit blocks form one of constituent elements on a recursive feedback pass, wherein a total number of the constituent elements formed with the first and second circuit blocks is odd including one and wherein a capacitor is connected to the differential-mode current output.
In the filter circuit, a plurality of at least the transconductor and the capacitor and the current supply are used and a given combination of these is mutually connected. The constituent element is not limited to the first circuit block and the second circuit block. Typically, the transconductor is referred to as Operational Transconductance Amplifier (OTA). In the specification, the term xe2x80x9cinputxe2x80x9d means xe2x80x9ca pair of input terminalsxe2x80x9d and the term xe2x80x9coutputxe2x80x9d means xe2x80x9ca pair of output terminals.xe2x80x9d
According to the first aspect of the filter circuit, the first circuit block is a transconductor which has a certain transconductance for the first differential-mode voltage input and has high negative gain for the common-mode voltage of the first differential-mode voltage input, and the second circuit block is an amplifier which has a predetermined gain for the second differential-mode voltage input and has a negative gain comparable to predetermined gain for the common-mode voltage of the second differential-mode voltage input.
Further according to the above filter circuit, the transconductor has a pair of first field effect transistors, connects a pair of sources of the first transistors to a common fixed-voltage terminal; a pair of gates of the first transistors are used as input terminals of the transconductor; a pair of drains of the first transistors are connected to a current source and are used as output terminals; and the amplifier has two pair of second field effect transistors having a pair of input side second transistors and a pair of output side second transistors; a pair of sources of the input side second transistors are connected to the common fixed-voltage terminal; a pair of gate of the input side second transistors are used as input terminals; a pair of drains of the input side second transistors are respectively connected to a pair of sources of the output side second transistors; a pair of gates and a pair of drains of the output side second transistors are connected to at least one common fixed voltage terminal.
Still further according to the above filter circuit, a filter frequency characteristic is proportionally adjusted by controlling all of the current sources with their mutual current ratio being constant.
Second aspect of the present invention is to provide a filter circuit comprising a transconductor, having a first differential-mode voltage input and a differential-mode current output, having a certain transconductance for the first differential-mode voltage input and having a high negative gain for a common-mode voltage of the first differential-mode voltage input, an amplifier, having a second differential-mode voltage input and a differential-mode voltage output, having a predetermined gain for the second differential-mode voltage input, and having a negative gain comparable to the predetermined gain for the common-mode voltage of the second differential-mode voltage input, a current source connected to the first differential-mode voltage output of the transconductor, and a capacitor connected to the first different-mode voltage output of the transconductor, wherein the transconductor and the amplifier forms a recursive feedback pass that makes a round of a pair of nodes, input and output terminals of the transconductor or the amplifier and the pair of nodes, and wherein a total number of the transconductor and the amplifier which pass through the feedback loop is odd including one.
According to the second aspect of the filter circuit, one transconductor and even number of the amplifiers exist in the recursive feedback pass.
Further according to the second aspect of the filter circuit, two transconductors and odd number of the amplifiers exist in the recursive feedback pass.
Alternatively, three capacitors may be connected to the two transconductors; one of the three capacitors may be connected to the differential-mode current output of one of the transconductors and two of the three capacitors may be connected between the fixed voltage terminal and the differential-mode current output of the transconductors.
Third aspect of the present invention is to provide a filter circuit comprising first, second, third and fourth transconductors, each having a differential-mode voltage input and a differential-mode current output, each having a certain transconductance for the differential-mode voltage input and each having a high negative gain for a common-mode voltage of the differential-mode voltage input, and an amplifier, having a differential-mode voltage input and a differential-mode voltage output, having a predetermined gain for the differential-mode voltage input and having a negative gain comparable to the predetermined gain for the common-mode voltage of the differential-mode voltage input, wherein the differential-mode current output of the first transconductor is connected to the differential-mode current output of the second transconductor via a first pair of nodes; the nodes are connected the differential-mode voltage input of the amplifier, and each node is connected to a capacitor and a current source, wherein the differential-mode current output of the third transconductor is connected to the differential-mode current output of the fourth transconductor via a second pair of nodes; each node is connected to a capacitor and a current source, and the nodes are connected to the differential-mode voltage inputs of the second and the fourth transconductors with being inverted the differential-mode current outputs of the third and the fourth transconductors, wherein the differential-mode voltage output of the amplifier is connected to the differential-mode voltage input of the third transconductor, and wherein the differential-mode voltage input of the first transconductor is used as a filter input.
Alternatively, the differential-mode voltage input of the amplifier may be connected to the differential-mode voltage input of the fourth transconductor, the differential-mode voltage output of the amplifier may be connected to the differential-mode input voltage of the second transconductor, and the differential-mode current output of the first transconductor may be connected to the differential-mode input voltage of the third transconductor.
Fourth aspect of the present invention is to provide a filter circuit comprising first, second and third transconductors, each having a differential-mode voltage input and a differential-mode current output, each having a certain transconductance for the differential-mode voltage input and each having a high negative gain for a common-mode voltage of the differential-mode voltage input, and an amplifier, having a differential-mode voltage input and a differential-mode voltage output, having a predetermined gain for the differential-mode voltage input and having a negative gain comparable to the certain gain for the common-mode voltage of the differential-mode voltage input, wherein the differential-mode current output of the first transconductor is connected to a pair of capacitors and a first pair of current sources via a first pair of nodes, the nodes are connected to the differential-mode voltage input of the amplifier, wherein the differential-mode current output of the second transconductor is connected to the differential-mode current output of the third transconductor via a second pair of nodes; the nodes are connected to the differential-mode voltage inputs of the first and the third transconductors with being inverted the differential-mode current outputs of the second and third transconductors, a capacitor is connected between the second pair of nodes, and a second pair of current sources is connected to the nodes, wherein the differential-mode voltage output of the amplifier is connected to the differential-mode voltage input of the second transconductor, and wherein the pair of capacitors is used as a filter input.
Alternatively, the differential-mode voltage input of the amplifier may be connected to the differential-mode voltage input of the third transconductor, the differential-mode voltage output of the amplifier may be connected to the differential-mode voltage input of the first transconductor, and the differential-mode current output of the first transconductor may be connected to the differential-mode voltage input of the second transconductor.
Fifth aspect of the present invention is to provide a filter circuit comprising first, second and third transconductors, each having a differential-mode voltage input and a differential-mode current output, each having a certain transconductance for the differential-mode voltage input and each having a high negative gain for a common-mode voltage of the differential-mode voltage input, and an amplifier, having a differential-mode voltage input and a differential-mode voltage output, having a predetermined gain for the differential-mode voltage input and having a negative gain comparable to the predetermined gain for the common-mode voltage of the differential-mode voltage input, wherein the differential-mode current output of the first transconductor is connected to a capacitor and a first pair of current sources via a first pair of nodes, the nodes are connected to the differential-mode voltage input of the amplifier, wherein the differential-mode current output of the second transconductor is connected to the differential-mode current output of the third transconductor via a second pair of nodes; the nodes are connected to the differential-mode voltage inputs of the first and the third transconductors with being inverted the differential-mode current outputs of the second and third transconductors, a pair of capacitor and a second pair of current sources is connected to the nodes, wherein the differential-mode voltage output of the amplifier is connected to the differential-mode voltage input of the second transconductor, and wherein the pair of capacitors is used as a filter input.
Alternatively, the differential-mode voltage input of the amplifier may be connected to the differential-mode voltage input of the third transconductor, the differential-mode voltage output of the amplifier may be connected to the differential-mode voltage input of the first transconductor, and the differential-mode current output of the first transconductor may be connected to the differential-mode voltage input of the second transconductor.
Sixth aspect of the present invention is to provide a high-order filter circuit comprising a combination of the above-described filter circuits.